Circuit building question

[Jimmy Yakapucci]

I have a question about a circuit that I am working on. I need to use a resistor in-line with an LED that I am using in order to drop the voltage from 5v down to 3.5v. My question is this: I would like to connect the resistor directly to the LED and in order to prevent a short, was contemplating just putting heat shrink tubing over the whole resistor. Would this be a problem for heat dissipation, or would there not be that much heat generated? Any ideas?

 

JY

[GroovyBee]

Resistors come in a watt rating as will as an ohms value. Work out how much power will be dissipated in the resistor (P=VxA). Then choose a resistor with a higher watt value than you need.

[roland p]

What current does the LED need?

Normal LEDs don't use much current, so there isn't much heat generated in the resistor, so a plain 0.25w resistor will be sufficient.

 

Here is some info about the theorie behind the resistor/led circuit: http://en.wikipedia.org/wiki/LED_circuit

 

 

My knowledge is a bit rusty but this is how you calculate it (I hope ):

If the current is 2mA (low current LED) and the voltage drop is 3.5 the power generated in the resistor is:

P=UxI

P=1.5 x 0.002 = 0.003 watt

Edited May 25, 2009 by roland p

[Jimmy Yakapucci]

Thanks for the replies, but my main concern is not it figuring out what wattage of resistor to use, but rather if there will be a heat problem if I enclose the whole resistor in heat shrink tubing. I figure that at 5 volts going in and the LED drawing a max of 40mA that there shouldn't be much heat, but I wanted to make sure before I melt something down.

 

JY

[CPUWIZ]

Thanks for the replies, but my main concern is not it figuring out what wattage of resistor to use, but rather if there will be a heat problem if I enclose the whole resistor in heat shrink tubing. I figure that at 5 volts going in and the LED drawing a max of 40mA that there shouldn't be much heat, but I wanted to make sure before I melt something down.

 

JY

 

Negative, I've done it dozens of time, it's not an issue.

[Jimmy Yakapucci]

Great! Thanks for the help. Now once I get moved, I'll have to sit down and start working on it. Just in case anyone cares what my plan is, it is to mod one of my Genesis Model 1s. I am going to do a different type of S-video mod where the signal will come out the DB9 ext port on the back so that I don't have to add another jack. I will also be routing stereo audio out of that port as well. All I will need to do is to mod an S-video/audio cable for the correct pin out. I am also going to add a region switch tied into a bi-color LED to replace the stock red one. It will light up one color for US and a different for Japanese.

 

JY

[MiKOTRON]

That sounds pertty cool! Good luck!